1. Field of the Invention
The present invention relates to the construction of electroluminescent elements in which light-emitting thin films are used which can be employed, for example, as displays for lap-top computers, televisions and mobile communications, and a method for their manufacture.
2. Description of the Related Art
Light-emitting elements in which the electroluminescence of organic compounds is employed are auto-luminescent and so they have a high visibility, since they are completely solid elements they have excellent impact resistance and since they are also characterized by a low drive voltage they are clearly useful as light emitting elements for various types of display equipment.
Clearly multi-color elements like those seen in Braun tubes (CRT) and liquid crystal displays (LCD) will be required to extend the range of application of the abovementioned organic EL elements as display elements.
The following methods, for example, were known as means of producing multi-color display apparatus using EL elements in the past. (1) Methods in which EL materials which emit light of the three colors red (R), green (G) and blue (B) are arranged in a matrix (Japanese Laid-open Patent Publications Sho.57-157487, Sho.58-145989 and H3-214593 for example). (2) Methods in which the three primary colors RGB are realized by combining color filters with EL elements which emit white light (Japanese Laid-open Patent Publications H1-315988, H2-273496, H3-194895 for example). (3) Methods in which EL elements which emit blue light and fluorescent conversion elements are combined for conversion to provide the three primary colors RGB (Japanese Laid-open Patent Publication H3-152897).
However, the methods described under (1) above must have three types of light emitting material arranged in a very finely divided manner in a matrix and so the technology is complex and the product cannot be manufactured cheaply, and since the life expectancies of the three types of light emitting material will generally be quite different there is a further disadvantage in that there will inevitably be a displacement of the color balance with the passage of time. Furthermore, the methods described in (2) make use of some of the light output from EL elements which emit white light using color filters and so there is a disadvantage in that the utilization efficiency of the EL light, which is to say the conversion efficiency, is low. For example, the white EL light simply comprises the three primary colors RGB which are of equal intensity and if the colors are obtained from this using color filters then at the most only a 33% conversion efficiency can be obtained. In practice, when the emission spectrum and visual perception are taken into consideration then only a much lower conversion efficiency can be obtained. On the other hand, if the three primary colors RGB could be obtained with conversion efficiencies of more than 33% respectively with the methods described under (3) these would be better methods than those described under (2) above.
Thus, methods in which fluorescent conversion films are arranged in the lamination direction over the EL elements and the EL emission color is changed variously are known (Japanese Laid-open Patent Publications Sho.63-18319, H3-152897). Since blue from among RGB is emitted by the organic EL elements themselves this should be used. In this case, the conversion efficiency can be taken to be 100%. Furthermore, an 80% conversion efficiency can be achieved using coumarin 153 as disclosed in Japanese Laid-open Patent Publication H3-152897 for green. Furthermore, a method of converting EL element blue light to red with a conversion efficiency of more than 33% has been disclosed in Japanese Laid-open Patent Publications H8-286033.
Thus, the fluorescent conversion method is superior as a means of achieving a full color display, but in terms of the actual method of manufacture, processes similar to the those for the color filters which are used conventionally in color liquid crystal display apparatus are required to manufacture a fluorescent conversion film and there is a problem in that this is very expensive.
The present invention is intended to overcome these weaknesses in the conventional technology. An object of the present invention is to provide electroluminescent elements with which the emitted light of a blue light emitting organic EL element can be converted to other colors with a high conversion efficiency of at least 33% and to provide a method of manufacture whereby these color electroluminescent elements can be manufactured cheaply using an ink-jet method.
The electroluminescent elements of this invention are characterized in that they comprise a light emitting layer comprised of a fluorescent first compound which is arranged between a cathode and an anode, and a hole injection transport layer comprising a mixture of a second compound which absorbs the fluorescence generated by the aforementioned first compound and emits a longer wavelength that the aforementioned fluorescence and a compound which has a charge injection/transportation capacity which is arranged between the aforementioned anode and the aforementioned light emitting layer. By this means the second compound layer is arranged on the light output side and so all of the light emitted from the light emitting layer which is formed by the first compound falls on the second compound layer, is absorbed by the second compound and discharged after being wavelength converted, and so the color purity is high.
If, in this case, the construction is such that the concentration of the aforementioned second compound has a gradient between the aforementioned light emitting layer and the aforementioned hole injection transport layer then movement of the holes is facilitated and the light emission efficiency is improved.
Furthermore, another electroluminescent element of the invention is characterized in that it is comprises a light emitting layer having a mixture of a fluorescent first compound and a second compound which absorbs the fluorescence emitted by the aforementioned first compound and emits fluorescence of longer wavelength that the aforementioned fluorescence which is a arranged between an anode and a cathode, and in that it is established in such a way that the concentration of the aforementioned second compound with respect to the aforementioned first compound in the aforementioned light emitting layer varies with a gradient along the thickness direction of said light emitting layer. Moreover, such elements are characterized in that the proportions of the aforementioned first compound and the aforementioned second compound are from 99.9:0.1 to 90:10. With such constructions, the charge injected from the electrodes reaches the light emitting layer with good efficiency and so the light emitting layer comprising mainly the first compound emits the fluorescence of the first compound and then this fluorescence and the fluorescence which has been reflected by the cathode is absorbed directly and indirectly with good efficiency by the second compound and the second compound emits its own fluorescence. In this case there is no distinct interface between the first compound and the second compound and so direct energy transfer occurs at the same time as the energy transfer by means of light and the conversion efficiency is improved.
Furthermore, another such element is characterized in that a charge injection transport layer is formed between the aforementioned light emitting layer and the electrodes in the aforementioned electroluminescent element. With this construction charge injection in the aforementioned construction is achieved with greater efficiency with the result that the light emitting efficiency is also improved.
These elements are also characterized in that the first and second compounds used in the abovementioned electroluminescent elements are organic compounds or organometallic compounds. The drive voltage can be greatly reduced in this way.
These elements are also characterized in that the surface of the abovementioned electroluminescent element is subjected to an anti-glare treatment and/or anti-reflection treatment. The reflected light from the surface of the electroluminescent element can be reduced or scattered in this way and so the display becomes easier to see.
Moreover, these elements are characterized in that they are provided with a means whereby the layer structure of the electroluminescent element itself is isolated from the surroundings. Each layer is protected by this means and the durability is improved.
Next, the method for the manufacture of electroluminescent elements where a light emitting layer is sandwiched between an anode and a cathode is characterized in that it is provided with a process whereby a transparent anode is formed on a transparent substrate, a process whereby a hole injection transport layer is formed using a mixture of compound which has a charge injection/transportation function with a second compound which absorbs the fluorescence produced by a fluorescent first compound and emits fluorescence of a longer wavelength than said fluorescence, a process whereby said first compound is deposited as a film on the whole of the surface of the aforementioned hole injection transport layer using a solvent of which the compatibilities in respect of the aforementioned first compound and the aforementioned second compound are controlled and in which said second compound in the aforementioned hole injection transport layer permeates into said first compound layer and a light emitting layer is formed, and a process whereby a cathode is formed on said light emitting layer. With this method the hole injection transport layer can be patterned and so it is possible to avoid short circuiting between the anode and cathode even if substances which have good charge injection properties which have a high electrical conductivity are used for the hole implanting material. Furthermore, the fluorescent conversion substance can also be patterned at the same time and so full color and high efficiency electroluminescent elements can be manufactured using the best materials with a simple process.
Next, there is a method for the manufacture of electroluminescent elements in which a light emitting layer is sandwiched between an anode and a cathode in which there are provided a process whereby a transparent anode is formed on a transparent substrate, a process whereby a film of the first fluorescent compound is formed over the whole surface, a process whereby a second compound which absorbs the fluorescence emitted by a fluorescent first compound and emits fluorescence of a longer wavelength that said fluorescence is applied as a solution, said second compound is caused to permeate into the aforementioned first compound and a light emitting layer is formed, and a process whereby a cathode is formed on the aforementioned light emitting layer.
Furthermore, there is a method of manufacture wherein there are provided a process whereby a transparent anode is formed on a transparent substrate, a process whereby a second compound which absorbs the fluorescence emitted by a fluorescent first compound and emits fluorescence of a longer wavelength than said fluorescence is applied as a solution, a process whereby the aforementioned first compound is formed as a film over the whole surface using a solvent of which the compatibilities in respect of the aforementioned first compound and the aforementioned second compound are controlled and said second compound permeates into said first compound and a light emitting layer is formed, and a process whereby a cathode is formed on the aforementioned light emitting layer. By means of these methods of manufacture it is possible to achieve easily changes in the emission color of adjoining picture elements and to reduce the manufacturing costs. Furthermore, in those cases where the first compound is discharged from an ink-jet head, the concentration gradient in the thickness direction can be controlled by controlling the compatibility with the second compound. It is possible by this means to manufacture electroluminescent elements in which the gradient is matched to the characteristics of the first compound and the second compound.
Next, there is a method for the manufacture of electroluminescent elements in which a light emitting layer is sandwiched between an anode and a cathode in which there are provided a process whereby a transparent anode is formed on a transparent substrate, a process whereby a light emitting film is formed by mixing a fluorescent first compound and a second compound which absorbs the fluorescence emitted by said first compound and emits fluorescence of a longer wavelength than said fluorescence and the mixture is applied as a solution, and a process in which a cathode is formed on the aforementioned light emitting layer. With this method it is possible to manufacture full color electroluminescent elements with a very simple process and very cheaply.
Next, there is a method for the manufacture of electroluminescent elements in which a light emitting layer is sandwiched between an anode and a cathode in which there are provided a process whereby a transparent anode is formed on a transparent substrate, a process whereby a light emitting film is formed by attaching a fluorescent first compound as a solution, and a process whereby a cathode is formed on the aforementioned light emitting layer. With this method it is possible to manufacture full color electroluminescent elements with a very simple process and very cheaply.
A process whereby a hole injection transport layer is formed between the anode and the light emitting layer may be provided in the electroluminescent elements as a means of resolving the problems described above. It is possible to manufacture bright elements in which charge implantation is achieved efficiently by forming a hole injection transport later.
There is a method of manufacture in which the application of the abovementioned solution in an electroluminescent element is carried out by discharging said solutions onto the surface to which they are to be attached using an ink-jet system as a means of resolving the problems described above. With an ink-jet system, fluid compounds can be introduced selectively into each picture element without wasting material.
There is a method wherein banks are formed between picture elements in order to divide the aforementioned picture elements. In this way it is possible to prevent cross-contamination between adjacent picture elements when forming a film with the ink-jet method and to prevent diffusion of the organic molecules between adjacent picture elements after the elements have been produced. Furthermore, there is no crossing of the emission colors between picture elements and brilliant light emission can be achieved.